For the preparation of compression-moulding compositions use is generally made of phenol-formaldehyde resins produced by condensation of phenol and formaldehyde in an acidic medium and a filler such as that of an organic origin. Among organic fillers use is predominantly made of wood dust, i.e. finely divided wood of coniferous varieties. The filler has a marked influence on mechanical properties of the final articles and to a lesser extent--on heat-resistance and electrical properties.
The need for a better quality of the final articles manufactured from compression-moulding compositions as well as for an enlarged range of raw materials for their preparation gives rise to an intensive research work which has been carried out in different directions such as modifications of a binder, various kinds of treatment of purpose additives and fillers.
Compression moulding compositions with properties thereof modified by way of incorporation, into a novolac resin (or compression-moulding composition), of polyvinyl alcohol or a polyhalohydrocarbon of the paraffin type, or alkyl compounds of titanic acid, or urea and sodium salt of sulphonated lignin are known in the art (cf. FRG Pat. No. 1,669,891; Japanese Patent Applications Nos. 48-17379, 51-17997; U.S. Pat. No. 3,790,441).
Also known are compression-moulding compositions based on amine-modified novolac resins and aromatic polycarboxylic compounds or thermosetting compositions including resins, a curing agent, a solvent, additives, or phenol-lignine-formaldehyde resins (cf. U.S. Pat. No. 3,756,980; FRG Pat. No. 2,141,002; USSR Inventor's Certificate No. 496,290).
There have been described compositions featuring an increased curing speed. A material based on such compositions also retains good processability which is attained by the use of rapid-curing resins or a mixture of hexamethylenetetramine with water-absorbent agents accelerating the curing process.
A compression-moulding composition possessing an increased curing speed while retaining high mechanical properties is also known in the art. It contains 7 to 30% of a resol resin per 100% of a novolac resin, aromatic carboxylic acids in an amount of from 0.4 to 4 parts by weight per 100 parts by weight of the resin mixture, saw dust and other conventional additives.
There are compositions possessing improved dielectric properties which incorporate a copolymer based on polycapramide, phenol and formaldehyde or a mixture of a powder-like polytetrafluoroethylene, glass fibres, china clay, fluorspar.
There are also compression-moulding compositions based on a phenol-formaldehyde resin and incorporating a filler pretreated with a phenol-formaldehyde resin or with a metal salt of an aliphatic C.sub.3 -C.sub.20 acid, or with unsaturated aldehydes; alternatively, a mixture of fillers is used in such compositions.
Known in the art are compression-moulding compositions containing as a filler a mixture of textolite saw-dust, cotton floss, carbonized cotton cellulose previously impregnated with a phenol-formaldehyde resol resin, or a heat-treated turf preliminary extracted with organic solvents, or a heat-treated non-hydrolyzable residue of turf hydrolysis products, or a crushed root of bistort after extraction of tannides therefrom or a finely-ground furfurol bottoms.
Also known in the art are compression-moulding compositions containing, as a filler, crushed nut shells or a solid lignine (lignosulphonates or mixtures thereof) or lignocellulose; as an additive to a conventional filler use is made in such compositions of up to 40% by weight of alkali-extracted bark or rice scale.
Despite a large scale of research work on elaboration new sources of raw materials for organic fillers and modification of other ingredients of compression-moulding compositions in order to improve their properties, in the manufacture of compression-moulding compositions based on phenol-formaldehyde resins wood flour has been preferably used hitherto as a filler.
Such compression-moulding compositions also incorporate curing agents, curing accelerators, lubricants, dyes. As a curing agent for such resins use may be made of additional amounts of an aldehyde, preferably formaldehyde. Hexamethylenetetramine is also incorporated into such compositions for the same purpose.
As accelerators of the curing process the composition contains, e.g. oxides of calcium or magnesium.
A lubricant is incorporated to prevent adherence of the articles to the mould. As a lubricant use is generally made of stearin or stearates of magnesium, calcium, zinc.
Dyes are incorporated into such compositions to impart a required colour to the final articles.
Nigrosine and induline are generally used as the dyes.
The above-described prior art compositions feature a full spectrum of valuable properties such as high mechanical properties, good dielectric properties, high heat-resistance; however, they have essential disadvantages. Thus, said compositions feature an insufficient plasticity, rather long curing time (low curing rate). The latter characteristic has a detrimental effect on processability of compositions from the economic standpoint. The necessity of using wood flour of a very thin fineness substantially adds to the production cost of the resulting compression-moulding composition. Furthermore, the use of wood flour does not enable the production of compression-moulding powders with a sufficiently high water-resistance.
The use of wood flour preliminarily treated with phenolic alcohols (phenolic wood flour), phenol-formaldehyde resins or with other compositions results in an increased water-resistance of the final articles; however, for a shorter curing time of compression-moulding compositions of this type it is necessary to employ a modified (by various methods) binder, pre-treated purpose additives, and the like. All this complicates the process of the preparation of compression-moulding compositions and increases their production costs.